1. Field of the Invention
This invention relates to an ultrasonic grinder system for tuning the frequency of a ceramic filter and which is suitable for automating the tuning step. Further, the present invention relates to a method for effective trimming to obtain a finely tuned ceramic filter using the ultrasonic grinder system.
2. Brief description of the related art
Recently, in the mobile communication technology, a 800 MHz band single body ceramic filter has been commonly featured in small telephone products. For example, U.S. Pat. Nos. 4,431,977 and 4,742,562 disclose such ceramic filters made by a single ceramic block. Those ceramic filters are tuned by trimming a predetermined portion of a metallic layer metalized on the ceramic block.
In the tuning step, it is required to accurately remove the metallic layer in a predetermined pattern.
An example of such a trimming method is disclosed in U.S. Pat. No. 4,855,693 filed by the applicant. Further, trimming methods which feature different physical principles employ correspondly different types of trimming apparatus. For example, a laser trimming method features a high-power laser beam to evaporate the metallic layer on the ceramic and a sand blast method features a nozzle which blows sands of carbon silicate to cut the metallic layer. One of the most conventional trimming method features a micro rotary grinder, using a diamond point which directly cuts the metallic layer.
However, the above known trimming methods have respective disadvantages. For example, the laser trimming method needs a high electric power source to obtain high power energy of the laser beam and it is difficult to control unnecessary heat which may cause a crack of the ceramic.
As to the sand blast method, it is difficult to obtain an accurate depth and area of the removed portion because the sands of carbon silicate are too hard and also cut the nozzle itself. Therefore, the nozzle needs to be changed frequently otherwise the diameter of the cut area becomes large. Generally, according to the sand blast method, "try and check" (measuring filter characteristic during the trimming step) is necessary to obtain a fine tuned ceramic filter.
As to the micro rotary grinder method, because the diamond point is easily clogged with the powder of ceramic which is cut with the removed metallic layer, it is necessary to dress the diamond point frequently. Further, sometimes, the powder of diamond comes off the diamond point, and it is necessary to change the diamond point.
As a result of the aforementioned disadvantages, the above trimming methods are rather unsuitable for automating the tuning step.